When manufacturing a flat container having a thin-walled shell part by blow-molding with stretch or without stretch by using an injection-molded preform, the wall thickness between forward and backward surface sides and right and left surface sides of the shell part of the preform is adjusted to provide a difference in the heat capacity of the shell part. The wall thickness is adjusted by forming the outer side of the shell part into an oval or parallel plane or by using a flat core for an injection core and thereby forming the inner opposed planes of the shell part into parallel planes. However, the wall thickness of the shell part is mostly adjusted by a flat core because the wall thickness can be easily set.
However, in the case of a narrow-mouthed small flat container for eyewash or cosmetics, the inside diameter of a preform is also restricted. Accordingly, the injection core used for the preform is limited to small inner diameter, thus, it becomes very difficult to form both sides of such injection core flat because the injection core has double cooling passages. For this reason, a small flat container which can be manufactured by adjusting the wall thickness of the shell part of a preform by a flat core is limited.
The present invention is made in response to the above situation and its object is to provide a novel preform capable of manufacturing a narrow-mouthed small flat container which is thin in wall thickness and is well adjusted in thickness distribution and to provide a small flat container manufactured by using the preform. The object is attained by solving the limitations on wall thickness adjustment due to a flat core by providing a wall thickness difference (heat capacity difference) indispensable for flat-molding and by preventing the shell part to be blow molded from biasing the wall thickness in flatting.